Magnusson, Alexander Erik

Abstract [en]

Electric and hybrid cars is one of the technologies to reduce the uses of fossil fuel. What is common with an electric car and a hybrid car is the use of battery to store electrical energy. To sustain high performance, long lifetime and to keep high safety in an electric vehicle it’s very important to control the temperature of the battery cells. Therefore it’s important to have a sufficient and well-designed cooling system that can keep the battery cells within recommended temperature range when the car is driving.

In this thesis, battery cooling and battery heat generation in the KTH formula student car “EV12e” are simulated and analyzed. The first part is to modulate the heat production that can occur when driving the car at the formula student competition. The second part is analyzing predesigned air-cooling.

The “Thévenin Equivalent circuit” was used as battery model with a fixed value of cells internal resistance, the heat was approximated with Joule heating and the internal chemical heating was neglected. By using logged data from last year’s car “EV11e” on Silverstone 2015, a drive cycle could be estimated for EV12e with driving time and by using a parametric study of driving behavior. To simulate the airflow and heat transfer in cooling channels the software “COMSOL Multiphysics®”. Simplified geometric structure of the battery and cooling channels were imported to the software together with parameters specified from the car “EV12e”. This data was then used to simulate the temperature changes in the battery.

The result showed that the battery generates 0.8-1.4MJ heating at 500-900W, for driving time of 25 minutes and a specific driving behavior. When driving at 500W output power under outdoor temperature of 30 o C, the maximum temperature of the battery reaches 49 o C at the end of the race. If the driver uses the maximum theoretical power output, the internal heating can come up to 950W after 25 min driving and reaches temperature of 64 o C with the initial and outdoor temperature of 25 o C.

The pre-designed air cooling can manage to keep the battery temperature under the maximum allowable battery cell temperature with the outdoor temperature at 25 o C or lower. If the outdoor temperature is higher than 25 o C the driver will have to consider the battery temperature when driving and should avoid quick accelerations.